Tracking tunable system



A. BLOCH TRACKING TUNABLE SYSTEM Filed oct. '22, 1943 lNvENToR ALFREDaLoH //f/myfe A oRm-:Y

Aug. 12, 1947.-

nente-eerie 19.41* y '2,425,454

UNITED s'rA'rEsrArlI-:Nr omer:

Alfred Bloch, Wembley, England, assignor, by

mesncl assignments, to Haleltine Research, l jnc., Chicago, lll., acorporation oi' Illinois Application October 22, 1943, Serial No.507,284

In Great Britain October 23, 194? 9 Claims. (Cl. Z50-4:0)

This linvention relates to a tracking tunable with the tuning capacitor.By appropriate adsystem including aplurality of circuits individual-vjustment of the trimmer. capacitors, the dem ly tunable over differentpredetermined operatc intermediate frequency may be precisely obtaineding-frequency ranges in such a manner that a for at least two points inthe operating-frequency predetermined constant relation is maintained 5range of the system. However, at other points between a frequencycharacteristic of the circuits theintermediate frequency may deviateconsideras they are tuned over their respective operating ably above orbelow its desired value, which is frequency ranges. As used throughoutthis specan unsatisfactory limitation ot the described iication and inthe appended claims, the expresprior art system. Similar trackingarrangements Vsion frequency characteristic oi a circuit is inhave beenprOpOSed for tunable systems utilizing tended to include the operatingor resonant-freidentical tuning capacitors and other reactance quencycharacteristic of the circuit as lwell as means for establishing aninitial predetermined the wave-length characteristic correspondingfrequency difference between the tunable circuits. thereto. l* However,such arrangements are also subject to The most familiar application of atracking u, the limitation that the tracking is inexact over tunablesystem is to a commercial broadcast rethe-operating-irequency range ofthe system.

ceiver of the. superheterodyne type. In such an It is an object of thepresent invention, thereapplication, the tracking system generallyconif01e, t0 provide an improved tracking tunable prises one o r moreradio-frequency selector cirsystem which is not subject to one or moreof the I, cuits tunable over the broadcast range, thefreabOVe-mentionedllimitations of the prior'art arquency-determining circuit of theheterodyning lrangements;

oscillator tunable over a frequency range corre- It is another object ofthe invention to provide l sponding to that of the radio-frequencyselector l `iin improved tracking tunable system including circuits butdisplaced therefromin the frequency a' plurality of circuitsindividually tunable over Spectrum by an amount equal to the desiredmdiirerent predetermined operating-frequency termediate frequency of thereceiver, and a uniranges and in which a predetermined constant ccntrc1mechanism for simuuanecusly tuning the relation is maintained between afrequency charcircuits over their respective operating-frequencyacteristic of each circuit as the circuits are timed ranges. As is wellunderstood in the art, for over their respective operating-frequencyranges.

optimum performance it is necessary that thev 30 In accordance with theinvention, a tracking tunable circuits ofthe tracking system have suchtunable System including plurality 0f circuits tuning characteristicsthat the intermediate frel individually tunable over dincrentpredetermined quency of the receiver is maintained at a con-Operating-frequency ranges comprisesa rsttunstant predetermined valuefor all conditions of able circuit including a Ieactance means of atuning. f iirst type and a reactance means of a second type In prior arttracking tunable systems of one simultaneously adjustable to have suchvalues type, it is conventional practice to utilize a. tunas to tune therst circuit over a ilrst predetering reactor, such as a variablecapacitor of the mined operating-frequency range- The Systemrotary-plate type, in each tuned circuit, the re- 91S0 COmPrSeS a'Second tunable circuit includinsr actors having substantially identicallaws of vari- 40 a reactance'means of the aforesaid first type and ationand being mechanically coupled for simu1- a reactance means of theaforesaid second type taneous adjustment. Thev rotor of the variablewhich are simultaneously adjustable to have such capacitor included inone circuit is sometimes values as to tune the second circuit over asecond i angularly displaced by an amount necessary to predeterminedoperating-frequency range- Unlproduce the desired intermediatefrequency. control means are provided in the system for Thisdisplacement of one rotor causes the tuning Simultaneously tuning therst" and second cir-l reactors to operate on diilerent portions o! theircuits over their respective operating-frequency frequencycharacteristics and results in undesirranges. FurthermoreI the values ofthe reactance able variations ofthe intermediate frequency cf means ofthe ilrst and second circuits areso the receiver as the circuits aretuned over their proportioned and their laws of variation are soindividual operating-frequency ranges. To cordetermined that the rst andsecond circuits rect such undesirable vvariations of the interhavelsubstantially equal and constant ratios of mediate frequency, adjustabletrimmer capacitors inductance to Capacitance over their respective areusually inserted in one or more of the tunable operating-frequencyranges, whereby a predetercircuits, being arranged in series andparallel v56 mined constant relation is maintained between .including atracking tunable system in accordance with the invention; Fig. 2 is aschematic representation of a frequency measuring system including amodiiled form of the invention; while Figs. 3a and 3b compriseequivalent circuit diagrams utilized in explaining the modified form ofthe invention included in the system of Fig. 2.

embodiment, such flrstcircuit is provided by the first radio-frequencyselector circuit of amplifier Referring now more particularly to Fig. 1of the drawing, there is represented a complete modulated carrier-signalreceiver of the superheterodyne type embodying the invention. Thereceiver comprises. in cascade, an antenna-ground circuit III, II, aradio-frequency ampliner- I2 of one or more stages, a modulator I3 andan associated r oscillator I4, an intermediate-frequency amplifier I5 ofone or more stages, a detector and automatic amplification control or A.V. C. supply I6, an audio-frequency amplifier I1 of one or more stages,and a sound reproducer I8. Automatic amplification control, or A. V. C.,is obtained in a well-known manner by a unidirectional bias voltagederived from the unit I6 and applied to the control electrodes of one or,more of the vacuum tubes included in radio-frequency amplifier I2,modulator I3., and intermediate-frequency amplifier I5. All of theelements and assemblies illustrated in schematic form may be of suitableconventional construction and arrangement.

Neglecting for the moment that portion of the I2. This selector 'circuitincludes a variable re-A actance means of a first type comprising avariable inductor and a variable reactance means of a. second typecomprising a variable capacitor 2|. The reactance means of the selectorcircuit are simultaneously vadjustable by unicontrol means. as indicatedby broken line 22, to tune the circuit over a first predeterminedoperatingfrequency range, this range corresponding to that portion ofthe broadcast frequency range to which the receiver is designed torespond. A pair of terminals 22, 24 are included in the selector circuitfor coupling the selector to the antennaground circuit I0, II andradio-frequency amplifying tube, these connections being omitted fromthe drawing for the sake of simplicity.

The tracking system also comprises a second tunable circuit including areactance means of the -aforesaid first type and a reactance means ofthe aforesaid second type simultaneously addescribed circuit whichembodies the present in-` vention, the circuit constitutes, in general,a conlventional superheterodyne receiver including automatic volumecontrol The operation of such a receiver being well understood in theart, a detailed description thereof is deemed unnecessary herein. Inbrief, however, signals inter-l justabie to tune the second circuit overa'second predetermined operating-frequency range. This circuitconstitutes the frequency-determining circult of the heterodyningoscillator Il and includes a variable inductor 25 and a variablecapacitor 26 which are simultaneously adjustable by unicontrolmechanism, as indicated by broken line 21, to tune the second circuitover its operating -`frequency range. 'I'he operating frequency rangefor this circuit, as well understood in thel art, is equal to that ofthe aforedscrlbed selector circuit, but is displaced therefrom in thefrequency spectrum by anamount equal to the desired intermediatefrequency of the receiver. The second circuit includes additionalreactance means of the aforesaid rst and second types, one beingconnected in parallel and the other being connected in series with thelike reactance means of the second circuit. As illustrated, suchadditional reactancemeans are provided by a fixed inductor 28 connectedin parallel with adjustable inductor 25 and a fixed capacitor 29connected in series with adjustable capacitor 26. Terminals 30 and 3lare connected to the described second circuit for the purpose ofcoupling this circuit to the oscillator tube of unit Il, theseconnections being omitted from the drawing.

sound reproducer I8 for reproduction. The automatic amplificationcontrol bias derived from unit I6 is effective to control theamplification of -one or more of the units I2, I3 and I5 to maintain theamplitude of the signal input to detector I6 within a relatively narrowrange for a wide range of received signal intensities.

Referring now more particularly to the portion of the circuit comprisingthe present invention, a tracking tunable system including a pluralityof circuits individually tunable over dinerent predeterminedoperating-frequency ranges is provided for maintaining theintermediate-frequency of the receiver at a substantially constantpredetermined value as the circuits of the tracking system are tunedover their respective operating-frequency ranges. The tracking systemFinally, the tracking tunable system includesv a unicontrol means,indicated by broken line 32,

for simultaneously tuning the first and second circuits over theirindividual operating-frequency ranges.

In order that the intermediate frequency of the receiver may bemaintained at a constant predetermined value for all conditions oftuning, the

values of the reactance means included in the iirst and second circuitsare so proportioned and their laws `of variation so determined that thefirst and second circuits have substantiallyequal and 'constant ratiosof inductance to capacitance (L/C ratios) over their respectiveoperating-frequency ranges. To satisfythese conditions in the Fig. 1arrangement, variable inductor 25 and variable capacitor 28 of thesecond circuit areselected to have values and laws of variationsubstantially identical to the corresponding reactance means '20 and 2|,respectively, of the iirst circuit. Further, the variable inductor andvariable capacitor of each of the circuits\are selected to have suchlaws oi variation that its ratio of inductance to capacitance (L/C`ratio) over its particular operating-frequency range is dened by thefollowing equation: A f

where n=ns=m (mduetanee in henriee), C=Cz1=Cna (capacitance in i'arads)and K is a constant. Y

Also. iixed inductor 28 and nxed capacitor 29 of thesecond circuit areselected, respectively, to have the values of inductance 'andcapacitance lgiven by the following expressions:

While in the foregoing discussion mention has been made of but a singleselector circuit of radiofrequency ampliiler I2, it will be understoodthat all such tunable selectors of the amplifier may be constructed asdescribed and coupled to unicontrol mechanism 32 to be tunedsimultaneously in a conventional manner.

The described tracking system is eiective to produce an intermediatefrequency of Ja. substantially constant predetermined value even thoughthe aforementioned constant ratio of inductance to capacitance'is notstrictly maintained in its Imi- 2) CFL 3) where L, C and K are thequantities indicated above, and eo/2r is the desired intermediate fre'-25 quency lo of the receiver.

When the several elements ofthe ilrst and second circuits have beenselected and proportioned as particularly described, the ratio ofinductance to capacitance for each o! the circuits over its respectiveoperating-frequency range is expressed by Equation 1. A

- Subject to the usual assumption that the resistances of the first andsecond circuits are negligible, it may be shown that the describedtrack'- ing tunable system of the invention maintains the intermediatefrequency fo of the receiver constant for all conditions of tuning. Inproving this relationshp, let w/21r be the resonant frequency f oir theilrst circuit and v1/2r be the resonant frequency f1 of the secondcrcuit. Then tunable circuits. Suppose, for example, that the totalivariation in -capacitivereactance with tuning s where Ce is thatcomponent of the capacitive reactance which causes theinductance-to-capacitance ratio of the circuit to be a constant K. asindicated in Equation 1.

If A] is the variation of the intermediate frequency from its desiredpredetermined value fo, it can be shown that having input terminals 4l,I2 for receiving a signal whose frequency is to be determined. Connectedin cascadewith radio-frequency ampliiier 40 are a modulator 43 and anassociated calibrated oscillator, a wide-band amplifier Il, and afrequency meter 4B. The elements of the system having a schematicrepresentation may be of By substituting in Equation 5 the values ofLas, Ln, Cas and`C2o from Equations 2, 3 and 6, it will be seen thatTherefore, the resonant frequencies f and f1 of 65 the iirst and secondcircuits, respectively, are

maintained at a constant-frequency diilerence fo as the ilrst and secondcircuits are tuned over their respective operating-frequency ranges.

Thus, n win be apparent that the tracking tunany conventionalarrangement and construction.

vductor 28' is connected in series with variable inductoi` 25', whilexed capacitor 29' is connected in parallel with variable capacitor 26 inthe second circuit of the system ,under consideration?- This resultsfrom the fact that the tunable circuits of the tracking system of Fig. 2are derived as image circuits of corresponding circuitssineluded in thetracking system of Fig. 1, as described hereinafter.

The derivation of an image circuit is illustrated by the circuitdiagrams of Figs. 3a and 3b. lThe circuit of Fig. 3a com-prises aninductor L. having a value oi.' :r henries connected in series with acapacitor C. having a value of y farads. The image of vthis circuit.represented in Fig, 3b, is derived by (a) replacing inductor L. by acapacitor Cs' having a value of l vhenries and (c) maintaining theelements of the image circuit in the same series or parallelrelationship 75 which their counterparts have in the main cirvfrom thefollowing consideration:

It may be readily demonstrated that, with the values of the inductormeans and capacitor means of the first and second circuits of thetracking system under consideration proportioned in accordance withEquations 12 to 15, inclusive, and with their laws of variation asdefined by 8 i circuit of modulator 43 selects the diuerence frequencyof the signals applied thereto from umts 40 and 44 and the signal thusderived. after amplification in unit 45, is applied to frequency meter46. IKnowing the calibrationof oscillator 44 and the constantwave-length difference established between the operatingI frequencies ofampli- Equation 1, the ratios of inductance to capacitance of the firstand second circuits over their respective operating-frequency ranges are'constant and equal to K2. By virtue. of these relationships, the wavelengths corresponding to the resonant frequencies of the first andsecond circuits of the modified tracking system are maintained at asubstantially constant wave-length difference as these circuits aretuned over their respective operating-frequency ranges.

This wave-length relationship may be seen Through the use of Equation 11the resonant frequency of the first circuit may be written as,

f'Tif (16 and the wave length corresponding to this frequency is,

where c=the velocity of propagation' of light.

The resonant frequency of the second circuit may be written as,

f1 4 12Kl f+f (18) and the corresponding wave length is,

1=41r2K1C(+fo) (19) Therefore, the wave lengths corresponding to theresonant frequencies of the first and second circuits are maintained ata substantially con-y stant difference equal to,

M=41r2KiCfo (20)y In considering the operation of thefrequencydetermining system of Fig. 2, it will be understood that theselector circuits of radio-frequency ner 40 and oscillator 44. meter 48may be calibrated to provide a direct indication of the frequency of thesignal applied to terminals 4I, 42.

A suitable expression for calibrating meter in terms of the signalapplied to terminals 4I, 42 is as follows:

f=[o`m] 21 where fo' is the intermediate frequency obtained in theoutput circuit of modulator 43 for a sianal applied to terminals 4I,length A.

It will be apparent that the system of Fig. 2

permits the frequencies of high-frequency signals applied to terminals4|, 42 to be determined through the use of a. frequency meter 46 whichmay have a muchlower range of operatingfrequencies. 'g

A tracking tunable system in accordance with the Fig. 1 embodiment ofthe invention maintains a constant predetermined frequency differlencebetween the resonant frequencies of the first and second circuits asthose circuits are tuned over their individual operating-frequencyranges. A tracking system -in accordance with the Fig. 2 embodimentmaintains aa constant predetermined wave-length difference between theresonant frequencies of these circuits over their respective tuningranges. Therefore, with each arrangement a predetermined constantrelation is maintained between a frequency characteristic of each of thefirst and second circuits as such circuits areA tuned over theirrespective operatingfrequency ranges. i

In the described preferred embodiments of the invention the secondcircuit of the tracking system includes a single adjustable inductor anda single adjustble'capacitor for tuning the circuit over itsoperating-frequency ran e. However, it will be understood that those eements which have been indicated as havinga fixed reactance may likewisebe adjustable for the purpose of tuning so long as the ratio ofinductance to capacitance of the first and second circuits aremaintained at the same constant-value for all conditions of tuning.` f

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will pe obvious to thoseskilled in the art that various changesand modifications may be madetherein without departing from the invention. andit is. therefore, aimedin the appended claims to cover all such changes and modifications asfall within cluding a reactance means of said rst type and a reactancemeans of said second .type simul- 42 having a wave taneously adjustableto have such values as to tune said second circuit over a secondpredetermined operating-frequency range, and unicontrol means forsimultaneously tuning said rst and second circuits over their respectiveoperating-frequency ranges, the values of' said reactance means of saidfirst and second circuits being so proportioned and their laws ofvariation being .so determined that said first and second circuits havsubstantially equal and constant ratios of ind ctance to capacitanceover their respective operating-frequency ranges, whereby apredetermined constant relation is ,maintained between a frequencycharacteristic of each of said first and second circuits as said firstand second circuits are tuned'over their respective operatingfrequencyranges.

2. A tracking tunable system including a plurality'of circuitsindividually .tunable over different predetermined operating-frequencyranges comprising, `a first tunable circuit including a reactance meansoi. a ilrst type and a reactance means of a second type simultaneouslyadjustable to have such values as to tune said first circuit over e,first predetermined operating-frequency range, a second tunable circuitincluding a reactance means of said first type and a reactance means ofsaid second type simultaneously adjustable to have such values as totune said second circuit over a second predetermined operating-frequencyrange, and unicontrol means for simultaneosly tuning said rst and secondcircuits over their respective operating-frequency ranges, the laws ofvariation of said reactance means of said first and second circuitsbeing substantiaily identical and their values-being so proportionedthat said first and second circuits have substantially equal andconstant ratios of inductance to capacitance over their respectiveoperating-frequency ranges, whereby a predetera frequency characteristicof each of said first and second circuits as said first and secondcircuits are tuned over their respective operatingfrequency ranges.

3. A tracking tunable system including a plurality of circuitsindividually tunable over dif ferent predetermined operating-frequencyranges comprising, a rst tunable circuit including a reactance means ofa, first type and a reactance means of a second type simultaneouslyadjustable to tune said first circuit over a first predeterminedoperating-frequency range, 'a second -tunable circuit including a,reactance means of said first type and a reactance means of said secondtype simultaneously adjustable to tune lsaid second circuit over asecond predetermined operating-frequencyrange, an additional reactancemeans of one of said types connected in parallel withthe like reactancemeans of said second circuit, an additional reactance means of the otherof said types connected in series vwith the like reactance means of saidsecond circuit. and unicontrol means for simultaneously tuning saidfirst and second circuits over their respective operating-frequencyranges, thegvalues of said reactance means of said first and secondcircuits being so proportioned and their laws of variation being sodetermined that said first and second circuits have substantially equaland constant ratios of inductance to capacitance over their respectiveoperating-frequency ranges, whereby a predetermined constant relation ismaintained between a frequency characteristic of each' of said first andsecond circuits as said first l0 and second circuits are tuned overtheir respective operating-frequency ranges.

4. A tracking tunable system including a plurality of circuitsindividually tunable over dierent vpredetermined operating-frequencyranges comprising, a yfirsttunable circuitincluding a other of saidtypes connected in series with the4 like reactance means of said secondcircuit,v and unicontrol means for simultaneously tuning said first and.second circuits over their respective operating-frequency ranges, thevalues vof said reactance means of said first and second circuits beingso proportioned and their laws of variation being so determined thatsaid first and second circuits have substantially equal and constantratios of inductance to capacitance over their respectiveoperating-frequency ranges. whereby a predetermined constant relation ismaintained between a frequency characteristic of each of said first andsecond circuits as' said first and second circuits are tuned over theirrespective operating-frequency ranges. y

5. A tracking tunable system including a plurality of circuitsindividually tunableover different predetermined operating-frequencyranges comprising, a first tunable circuit including a variablereactance means of a first type and a variable Ireactance means of asecond type simul-l taneously adjustable to tune said first circuit overay first predetermined operating-frequency range,

a second tunable circuitincluding a variable reactance means of saidfirst type and a variable reactance means of said second typesimultaneously adjustable to tune said second circuit over a secondpredetermined -operating-frequency range and having substantiallyidentical values and laws of variation as said reactance means of saidfirst and second types respectively of said rst circuit, an additionalfixed reactance means of one of said types connected in parallel withthe like ,reactance means of said second circuit,

i an additional fixed reactance means of the other ofv said typesconnected in series with the like reactance means of said secondcircuit, andunicontrol means for simultaneously tuning said first andsecond circuits over their respective operating-frei'iuency ranges, saidvariable reactance means of said first and second circuits having suchlaws of variation and said fixed reactance means of said second circuitbeing so proportioned that said first and second circuits havesubstantially equal and constant ratlos oflinductance to capacitanceover their respective operating-frequencyl ranges, whereby apredetermined constant relation is maintained between a frequencycharacteristic of each of said first and second circuits as said firstand second circuits are tuned over their respectiveoperating-freqlleIlCy I'aI'igGS.

6. A tracking tunable system including a plurality of circuitsindividually tunable over different predetermined operating-frequencyranges comprising. a iirst tunable circiut including inductor means andcapacitor means simultaneously adjustable to tune said rst circuit overa first predetermined operating-frequency range, a second tunablecircuit including inductor means and capacitor means simultaneouslyadjustable to tune said second circuit over a second predeterminedoperating-frequency range, an additional inductor means connected inparallel with said inductor means of said second circuit, an additionalcapacitor means connected in series with said capacitor means of saidsecond circuit, and'unicontrol means for simultaneously tuning saidfirst and second circuits over their respective operating-frequencyranges, the values of said inductor` means and capacitor means of s andsecond circuits being so proportioned'and their laws of variation beingso determined that said first and second circuits have substantiallyequal and constant ratios of inductance to capacitance over theirrespective operating-frequency ranges, whereby the resonant frequenciesof said ilrstv and second circuits are maintained at a substantiallyconstant-frequency difference as said rst and second circuits are tunedover their respective operating-frequency ranges.

7. A tracking tunable system including afplurality oi' circuitsindividually tunable over difierent predetermined operating-frequencyranges comprising, a first tunable circuit including a variable inductorand a variable capacitor simultaneously adjustable to tune said circuitover a first predetermined operating-frequency range, a second tunablecircuit including a variable inductor and a variable capacitorsimultaneously adjustable to tune said second circuit over a secondpredetermined operating-frequency range and having sulfostantiallyidentical values and laws of variation as said variable inductor andvariable capacitor respectively of said first circuit, an additionalfixed inductor connected in parallel with` said variableinductor-of.said second circuit, an additional fixed capacitor connected in serieswith said variable capacitor of said second circuit, and unicontrolmeans for simultaneously tuning said first and second'circuits overtheirY respective operating-frequency ranges, said variable inductorsand variable capacitors of said first and second circuits having suchlaws of variation that their ratios of inductance to capacitance overtheir respective operating-frequency ranges equals a constant K2 andsaid fixed inductor and fixed capacitor of said second circuit havingthe values of K/wo and i/Kwo respectively where wo/21r is the desireddif-f ference frequency fo ofthe resonant-frequencies of said rst andsecond circuits, whereby the resonant frequencies of said first andsecond circuits are maintained at said difference frequency fo as saidfirst and second circuits are tuned over their respectiveoperating-frequency ranges.

l 8. A tracking tunable system including a plurality of circuitsindividually tunable over different predetermined operating-frequencyranges comprising, a flrst tunable circuit including inductor means andcapacitor means each simultaneously adjustable to tune said firstcircuit over a rst predetermined operating-frequency range, a secondtunable circuit including inductor means and capacitor means eachsimultaneously adad first,

, l2 iustable to tune said second circuit over a second predeterminedoperating-frequency. range, an additional inductor means connected inseries 'with said inductor means of said second circuit.

. an additional capacitor means connected in parallel with'saidcapacitor means ofsaid second circuit, and unicontrol means forsimultaneously l tuning said ilrst and second circuits over theirrespective operating-frequency ranges, the values of said inductor meansand capacitor means of said first and second circuits being soproportioned and their laws of variation being so determined that saidiirst and second'circuits have substantially equal and constant ratiosof inductance to capacitance .over their respective operating-frequencyranges, whereby the wave lengths corresponding to the resonantfrequencies of said iirst and second circuits are maintained at asubstantially constant wavelegth difference as said rst and secondcircuits are tuned over their respective operating-frequency ranges.

9. A tracking tunable system including a piurality of circuitsindividually tunable over differ- 25 ent predeterminedoperating-frequency ranges comprising, a flrst tunable circuit includinga variable inductor and a variable capacitor simultaneously adjustabletotune said first circuit over a first predetermined operating-frequencyrange, a second tunable circuit including a variable inductor and avariable capacitor simull taneously adjustable to tune said secondcircuit over aA second4 predetermined operating-frequency range andhaving substantially identical values and laws of'variation as saidvariable inductcr and variable capacitor respectively cf said rstcircuit, an additional xed inductor conl nected in series with saidvariable inductor of said second circuit, an additional fixed capacitor40 connected in parallel with said variable capacitor of said secondcircuit, and unicontrol means for simultaneously tuning said first andsecond circuits over their respective operating-frequency frequencyranges, whereby Vthe wave lengths corresponding to the resonantfrequencies of said rst and second circuits are maintained at asubstantially constant wave-length difference as said first and secondcircuits are tuned over their respective operating-'frequency ranges.

ALFRED BLOCH.

, REFERENCES CITED The following references are lof record in the illeof this patent: v

FOREIGN PATENTS Country Date Netherlands June l5 1936 Number

